1. Field of the Invention
The present invention relates to a thermal printer that forms an image by a thermal head having a plurality of heating elements, and more particularly to a device and a method for measuring resistance of the heating elements, for modifying image data so as to compensate for variations in resistance between the heating elements.
2. Background Arts
As well known, thermal printers may be classified into thermal transfer printers and thermosensitive type printers. The thermal transfer printers use an ink film and transfers ink from the ink film onto a paper by heating the ink film. The thermosensitive type printers heat a thermosensitive recording medium directly to record an image thereon. To heat the ink film or the thermosensitive recording medium, the thermal printer use a thermal head with a linearly arranged array of large number of heating elements. The heating elements are constituted of resistors connected in parallel to one another.
U.S. Pat. No. 4,734,704 (corresponding to JPA No. 61-213169) discloses a color thermosensitive printer that uses a thermosensitive color recording medium. The color thermosensitive recording medium has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer, which are formed atop another in this order from a base material. These thermosensitive coloring layers, hereinafter called simply as the coloring layers, have different heat sensitivities that become lower as the distance from the outside surface increases. Thus, the deeper the coloring layer, the higher coloring heat energy is required. Furthermore, the coloring layers may be optically fixed, each by electromagnetic rays of a specific wavelength range. Therefore, recording of a full-color image on the thermosensitive color recording medium is performed in the order from the top or outermost coloring layer to the inner coloring layer, while optically fixing the just recording coloring layer prior to recording the next coloring layer, so as to avoid double-recording.
Each heating element applies a different coloring heat energy to the thermosensitive color recording medium in accordance with a characteristic curve of each coloring layer, to form a color dot at a different density. As the coloring heat energy, first a bias heat energy is applied for heating the thermosensitive color recording medium up to a temperature above which a particular color begins to be developed. Next, a gradation heat energy is applied for developing the particular color at a designated density. The bias heat energy is a constant value determined for each color according to the heat sensitivity or characteristic curve of the individual coloring layer. Generally, the heating element is activated for several ms to several tens of ms (milliseconds) to apply the bias heat energy. On the other hand, in order to reproduce fine gradation, the gradation heat energy needs to be controlled with more accuracy, so activation time or power conduction time is controlled by several xcexcs (micro seconds) to several tens of xcexcs after applying the bias heating energy.
In spite of such a fine control of heating or conduction time of the heating elements, the consequent image cannot exactly reproduce the desired fine gradation unless all the heating elements of the same thermal head have a completely uniform resistance. This is because the heating elements generate different heat energies if they have different resistances, even while they are driven for the same time. However, the heating elements generally have variations of about 5% to 10% in resistance. Moreover, the resistance of each heating element varies with age and its recording history. For this reason, the printed images tend to have imperfections, such as chromatic unevenness.
In order to eliminate such undesirable phenomena, U.S. Pat. No. 5,469,068 (corresponding to JPA No. 6-79897) proposes a color thermosensitive printer that measures resistances of the respective heating elements, and modify image data based on the measured resistances so as to compensate for the variations in resistance. In this prior art, a capacitor with a known capacitance is fully charged and, thereafter, discharged through each individual heating element, while counting the time required to discharge the capacitor down to a constant voltage level, e.g. a half of a power source voltage. Since the discharge time is proportional to the resistance of the heating element, the resistance of each individual heating element is obtained based on the discharge time and the known capacitance.
Concretely, the resistance R of one heating element is calculated according to the following equations, assuming that it takes a discharge time T for discharging of the capacitor having a capacitance C from a predetermined discharge start voltage E to a predetermined discharge stop voltage Vref through the heating element. In the above prior art, Vref=E/2.
Vref/E=exp{xe2x88x92T/(Cxc2x7R)}xe2x80x83xe2x80x83(1)
R=xe2x88x92T/C/ln(Vref/E)xe2x80x83xe2x80x83(2)
If the capacitance C is a known value, it is possible to calculate the resistance R by measuring the discharge time T. Even where the capacitance C is an unknown value, it is possible to calculate the resistance R according to the following equations, by measuring a discharge time Ts required to discharge the capacitor through a reference resistor whose resistance Rs is known.
R=Rsxc2x7T/Tsxe2x80x83xe2x80x83(3)
T=xe2x88x92Cxc2x7Rxc2x7ln(Vref/E)xe2x80x83xe2x80x83(4)
As a device for measuring the discharge time, a counter circuit or a timer of a microcomputer is used. The counter circuit counts by a predetermined unit time to output a count corresponding to the discharge time. The microcomputer calculates the discharge time by multiplying the unit time by the obtained count. For the sake of accuracy, it is desirable to predetermine the unit time as short as possible. However, with a sufficiently short unit time, the counter circuit is required to count up to a large value where the discharge time to measure is relatively long. That is, an expensive counter circuit with a large bit number is needed. With an inexpensive counter circuit that has a small counter number, the unit time has to be so long that it is hard to achieve sufficiently accurate measurement of the discharge time.
Furthermore, not only the resistance R of the heating elements, but also the above discharge start voltage E, the discharge stop voltage Vref and the capacitance C of the capacitor have variations respectively. For instance, the variation in the discharge start voltage E is xc2x12% when E=20V, variation in the reference voltage is xc2x12% when Vref=19V, the variation in capacitance C is xc2x120%, and the variation in resistance R is xc2x11%. If these values are applied to the above equation (2), the discharge time T would have a variation of about +116%xcx9cxe2x88x9283%. On the other hand, in order to save time for the resistance measurement, it is desirable to set the discharge stop voltage Vref to be closer to the discharge start voltage E. However, as the ratio of the discharge start voltage E to the discharge stop voltage Vref approaches xe2x80x9c1xe2x80x9d, the variation in the discharge time T comes to be very large, because being affected by a natural logarithm in the equation (4). Also for this reason, it is necessary to set the unit time to be long enough for preventing overflowing of the counter circuit, in order to use an inexpensive counter circuit with a small bit number.
In view of the foregoing, an object of the present invention is to provide a device and a method for measuring resistances of heating elements of a thermal head of a thermal printer, that make it possible to measure the discharge time with accuracy and thus calculate the resistance of the heating element with accuracy based on the discharge time, without the need for an expensive counter circuit having a large bit number.
Another object of the present invention is to provide device and a method for measuring resistances of heating elements of a thermal head of a thermal printer, that eliminate influence of variations in the set voltages and save time for the measurement while maintaining resolution of a counter circuit that measures the discharge time.
According to the present invention, a method for measuring resistance of each individual of parallel connected heating elements of a thermal head comprises the steps of:
charging a capacitor up to a first voltage level, the capacitor being connected in parallel with the heating elements;
discharging the capacitor from the first voltage level to a second voltage level through one the heating element whose resistance is to be measured;
starting counting discharge time after a predetermined delay time from the start of discharging the capacitor through the one heating element;
calculating a discharge time of the capacitor from the first voltage level to the second voltage level based on the delay time and a count obtained by the counting; and
calculating a resistance value of the one heating element based on the calculated discharge time.
Starting counting the discharge time after the predetermined delay time makes it possible to count a remaining period of the discharge time by a small unit time and thus with a sufficient accuracy, without the need for an expensive counter with a large bit number.
The delay time is preferably equal to or slightly less than a shortest discharge time required to discharge the capacitor from the first voltage level to the second voltage level through one heating element having a smallest resistance among the heating elements. Since the delay time is predetermined, it is possible to clock the delay time at a longer interval than a unit time of the counting.
According to another aspect of the present invention, a method for measuring resistance of each individual heating elements of a thermal head by measuring discharge time of a capacitor through each of the heating elements, the heating elements and the capacitor being connected in parallel to one another, comprises the steps of:
measuring a first reference discharge time of the capacitor from a first discharge start voltage to a discharge stop voltage through a reference resistor having a known resistance, by counting with a counter;
judging whether the first reference discharge time satisfies a condition that is determined based on a bit number and a unit time of the counter;
determining, if the first reference discharge time satisfies the condition, a second discharge start voltage that is higher than the first discharge start voltage by a degree that the counter would not overflow;
measuring discharge time of the capacitor from the second discharge start voltage to the discharge stop voltage sequentially through the reference resistor and each of the heating elements;
measuring, if the first reference discharge time does not satisfy the condition, discharge time of the capacitor from the first discharge start voltage to the discharge stop voltage sequentially through each of the heating elements; and
calculating respective resistance values of the heating elements based on the measured discharge times through the reference resistor and each of the heating elements.
Since the discharge time is first measured from the predetermined lower discharge start voltage, and is compared to the condition determined by the bit number and the unit time of the counter, it comes to be possible to determine a discharge start voltage for the resistance measurement, while taking account of variations or setup tolerances in the discharge start voltage, the discharge stop voltage, capacitance of the capacitor, and resistance of the reference resistor. Therefore, this configuration is effective to measure the resistance in a short time, while making good use of the capacity of the counter and preventing the counter from overflowing.
According to the present invention, a resistance measuring device for a thermal head having an array of parallel connected heating elements which are heated by a voltage supplied from a power supply section, and transistors connected in series to the heating elements in one to one relation comprises:
a capacitor connected to the power supply circuit in parallel with the heating elements;
a switch connected between the power supply section and the capacitor to connect or disconnect the capacitor and the heating elements to or from the power supply section;
a control device for turning the switch ON to charge the capacitor up to a first voltage level, and then turning the switch OFF to discharge the capacitor through one of the heating elements while setting a corresponding one of the transistors ON;
a delay circuit that starts clocking a predetermined delay time with the start of discharging the capacitor;
a counter that starts time-counting when the delay circuit finishes clocking the delay time, and stops counting when voltage charged in the capacitor reaches a second voltage level, the counter counting by a unit time that is shorter than a clock interval of the delay circuit; and
a calculation device for calculating a resistance value of the one heating element based on a discharge time determined by adding the delay time to a time obtained by multiplying the unit time by a count of the counter.
According to another aspect of the present invention, a resistance measuring device for a thermal head having an array of parallel connected heating elements which are heated by a voltage supplied from a power supply section, and transistors connected in series to the heating elements in one to one relation comprises:
a capacitor connected to the power supply circuit in parallel with the heating elements;
a reference resistor having a known resistance and being connected in parallel to the heating elements;
a transistor connected in series to the reference resistor;
a switch connected between the power supply section and the capacitor to connect or disconnect the capacitor and the heating elements to or from the power supply section;
a control device for turning the switch ON to charge the capacitor and then turning the switch OFF to discharge the capacitor through one of the heating elements and the reference resistor while setting a corresponding one of the transistors ON;
a counter for measuring discharge time of the capacitor from the start of discharging till charged voltage in the capacitor reaches a predetermined discharge stop voltage;
a judging device for judging whether a first reference discharge time is less than a comparative value that is determined based on a bit number and a unit time of the counter, the first reference discharge time being measured by discharging the capacitor through the reference resistor from a predetermined first discharge start voltage to the discharge stop voltage;
a discharge start voltage determining device for determining a second discharge start voltage based on the first reference discharge time and the first discharge start voltage if the first reference discharge time is less than the comparative value; and
a calculation device for calculating a resistance value of the one heating element on the basis of discharge times measured relating to the reference resistor and the one heating element from the second discharge start voltage after the second discharge start voltage is determined, or on the basis of the first reference discharge time and a discharge time measured relating to the one heating element from the first discharge start voltage if the first reference discharge time is not less than the comparative value.